Temperature compensated multiple character electronic display

ABSTRACT

A temperature compensated multiple character electronic display is described. Each character position includes a multielement matrix controlled by a transistor. The collectors of all transistors of all matrices are common. The emitters of all transistors of a single character matrix are common, and are selectively connectable to ground so that the characters can be sequentially enabled. The corresponding elements of all character matrices are connected to a common control line and are actuated by a single character generator. The power sequentially applied to each character is controlled separately to maintain a uniform display under varying operating temperatures by sampling the voltage of the base-emitter junction of a transistor of the enabled character as a measure of its temperature, and then applying power to the character during the following display cycle that is related to the sampled temperature.

[ 51 Apr. 3, 1973 United States Patet 9] Canton Primary Examiner-John W.Caldwell Assistant Examiner-Marshall M. Curtis Attorney-Harold Levine,James 0. Dixon, Andrew Rene E. Grossman, Melvin Sharp,

Richards, Harris and Hubbard and V. Bryan Medlock,

[73] Assignee: Texas Instruments Incorporated,

Dallas, Tex.

[57] ABSTRACT A temperature compensated multiple character electronicdisplay is described. Each character cludes [22] Filed: May 3, 1971position ina multielement matrix controlled by a The collectors of alltransistors of all transistor.

m a D n .m a m D. D. A ms u d 1 ..m 0' e R. m. p A l 1 2 [63]Continuation of Ser. No. 788,249, Dec. 31, 1968,

matrices are common. The emitters of all transistors of a singlecharacter matrix are common, and are abandoned.

selectively connectable to ground so that the characters can besequentially enabled. The corresponding elements of all charactermatrices are connected to a 1 02 w w ne 4 3..., 2M B04 06% 4 u .0 n 4 nm h "c 0-! e "us I h C km .m.m UIF 11:] 2 8 555 [r.:l

common control line and are actuated by a single 219/506 178/30 89haracter generator. The power sequential] y applied to each character iscontrolled separately to maintain a uniform display under varyingoperating temperatures by sampling the voltage of the base-emitterjunction of a transistor of the enabled character as a mea- 29/5O6 sureof its temperature, and then applying power to ""219/506 the characterduring the following display cycle that is related to the sampledtemperature.

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ATTORNEY TEMPERATURE COMPENSATED MULTIPLE CHARACTER ELECTRONIC DISPLAYThis application is a continuation of application Ser. No. 788,249,filed Dec. 31, 1968, now abandoned.

This invention relates generally to electronic display systems, and moreparticularly relates to multicharacter electronic displays of thethermal type.

In thermal type electronic displays, particularly those used to print onthermally sensitive paper, it has been found necessary to maintain auniform temperature during successive display cycles in order to provideuniform printing density. Temperature compensation is particularlyimportant in multicharacter displays wherein, for example, a largenumber of character matrices are sequentially energized to print anentire line on a page. In addition to variations in the ambienttemperature, the rate at which each particular character matrix isenergized, the type of characters being printed, and the ambienttemperature to a lesser extent, all determine the temperature to whicheach matrix cools between print cycles. For example, the last fewcharacters of a line may not be used as much as the first few characterswhen printing a page. If the same amount of power were to be applied tothe printhead during a print cycle, the ultimate printing temperature ofthe matrix would be determined by the temperature at the beginning ofthe cycle. Of course, the beginning temperature increases sharply if theduty cycle of the character matrix increases.

In accordance with this invention, the temperature of each character issampled prior to the print cycle for the particular character, and thepower that is applied to the character matrix during the print cycle isadjusted in such a manner as to achieve a predetermined temperatureduring the print cycle. Another important advantage of the presentinvention is that the same circuitry used for selecting the character tobe printed is used to select the temperature monitoring means at thecharacter matrix.

The novel features believed characteristic of this invention are setforth in the appended claims. The invention itself, however, as well asother objects and advantages thereof, may best be understood byreference to the following detailed description of an illustrativeembodiment, when read in conjunction with the accompanying drawings,wherein:

FIG. 1 is a simplified plan view of a multicharacter electronic displayin accordance with this invention;

FIG. 2 is an enlarged perspective view of a portion of the displaydevice shown in FIG. 1;

FIG. 3 is a detailed circuit diagram of a temperature compensatedelectronic display in accordance with the present invention; and

FIG. 4 is a simplified block diagram of the circuit of FIG. 3.

Referring now to the drawings, and in particular to FIG. 1, threefour-character electronic display devices in accordance with the presentinvention are each indicated generally-by the reference numerals a, 10band 100. Each of the electronic display devices 10 includes fourcharacter matrices l2a-l2d. Each matrix includes a 5 X 7 array ofelements B 42 each of which is air isolated around its periphery andwhich is bonded to a ceramic slice 14 by a thermal insulating epoxylayer 16. The ceramic slice 14 is mounted on a metal heat sink 17.

As will presently be described, a transistor T,. with a series resistorR, in the collector branch is formed by a diffusion in the interior faceof each of the elements E -E that is adjacent the epoxy layer 16, andthin film circuits disposed on the interior faces of the semiconductordevice are used to interconnect the diffused devices into an integratedcircuit. For example, element E, in each of the characters includes atransistor T, and a resistor R (see FIG. 3), element E includes atransistor T and a resistor R and element E in each character includestransistor T and resistor R The collectors of all of the transistors T-T of a particular character are connected through the respectiveresistors to a common collector voltage supply line 18. All of theemitters of the transistors T -T of each of the characters 12a-12d areconnected to separate common emitter supply lines 20a-20d. Each of thecharacter matrices 12 may then be separately enabled by selectivelyconnecting the appropriate emitter supply line 20a-20d to ground. Onlyone of the emitter circuits 20a-20d of the three heads 10 is selected bya character select switch 22 at any one time, so that only one matrix isenabled at a time.

The base contacts of the transistors are of the corresponding elementsof all characters in the device are also common. For example, the basesof transistors T, of characters l2a-l2d of all display devices 10 in thesystem are connected to a common control line C the bases of alltransistors T are connected to control line C and the bases of alltransistors T are connected to a common control line C Of course, itwill be understood that the bases of all transistors T- T (notillustrated) would be connected to corresponding control lines C -C (notillustrated). The control lines C C, extend to a character generator 24which energizes those lines necessary to produce the desired characterby turning selected elements in the enabled character matrix on.

In the operation of such a system, the character select switch 22 wouldtypically scan from the left-hand character matrix to the right-handcharacter matrix in sequence by connecting the common emitter line 20,of the successive character matrices to ground. Then during the periodthat a particular character is thus enabled, the character generatorproduces the positive voltage levels on the control lines C -C necessaryto generate the desired character at the selected character position.The positive voltage on the control line C for example, would turntransistor T, on thus causing element E, to be heated by the powerdissipated in resistor R,. Those elements on which control lines are atground potential would remain turned off." For example, if control lineC remains at ground potential, transistor T of the enabled character,character 12a for example, remains off because the collector-basejunctions of transistors T of all the other characters prevent currentfrom flowing from the energized common collector supply voltage line 18through the collector-base junction of the inactive transistors T tocontrol line C and thus to the base of transistor T of the enabledcharacter.

In accordance with the present invention, a constant current source 26(see FIG. 4) is connected to control line C and supplies a constantcurrent during a very short sample cycle that precedes the print cycle.The

current passes along control line C and through the base-emitter diodeof the transistor T of the character enabled by the character selectswitch 22. For example, if the character select switch 22 connectscommon emitter line 20a of device b to ground so as to enable charactermatrix 12a of device 10b, the current injected on control line C by theconstant current source 26 will pass through the base-emitter diode oftransistor T of matrix 12a of device 10b because the emitter circuits oftransistor T of all other matrices are opened by switch 22. The voltageon control line C will then be related to the temperature of thebaseemitter diode of transistor T and thus to the temperature of theenabled matrix.

When a short sample pulse is applied to input 34, the voltage of line Cis amplified by amplifier 28 and the amplified voltage sampled and heldby circuit 30. The stored voltage is then applied to the noninvertinginput of an operational amplifier 32 during a print cycle pulse appliedto terminal 36 immediately following the sample pulse. During the printcycle pulse, a switch 38 turns a series regulator 40 on to apply avoltage to the common collector line 18 that has a magnitude inpredetermined relation to the voltage stored by the sample and holdcircuit 30 as a result of the feedback loop 41 to the inverting input ofamplifier 32. The character generator 24 is also activated during theprint cycle to produce a positive voltage on the appropriate controllines C -C to cause the desired character to be generated by the enabledcharacter matrix 12a.

The circuit shown in the simplified block diagram of FIG. 4 is shown ingreater detail in FIG. 3 wherein corresponding components are designatedby the same reference characters. The constant current source 26includes a transistor 42 which is turned on and off by a switch 44. Whenconducting, transistor 42 supplies a constant current as a result of thevoltage divider connected to the base so that the voltage on line C isdetermined by the offset voltage of the base-emitter diode of theenabled transistor T This voltage decreases with an increase intemperature at a rate of about 0.02V/ C.

The voltage on line C is applied to input 46 of the operationalamplifier 28, the offset of which is set by variable resistor 48 in theconventional manner. Thus, the output of the amplifier 28 isproportional to the voltage on control line C and is applied to the baseof transistor 52 through resistor 50, and to the base of transistor 54through resistor 50 and diode 55. Transistors 52 and 54 form acomplementary switching pair for charging and discharging storagecapacitor 64 when turned on. Diode 55 provides an offset voltage toeliminate the dead spot at the crossover voltage. Transistors 52 and 54are turned on by the complement of the sample pulse derived from theinverter 56 and applied to the base of switching transistor 58. Thisturns transistors 58, 60 and 62 off, enabling the complementary samplingtransistors 52 and 54 so that the output voltage of the amplifier 28will charge storage capacitor 64. The voltage on capacitor 64 is thenstored during the subsequent print cycle after the transistors 52 and 54are off.

The voltage stored on capacitor 64 is applied to the 65 noninvertinginput of amplifier 32. The output of am plifier 32 is passed through aZener diode 66 and diode 68 and applied to the input base of a pair oftransistors 70 and 72 of the series voltage regulator 40. When the printcycle pulse is applied to input 36, inverter 74 turns transistor 76off," thus enabling transistors 70 and 72 to be turned on by the outputof amplifier 32. The voltage applied to the common collector supply line18 is then maintained at a level in predetermined relationship to thevoltage sample and stored on capacitor 64 as a result of the feedbacknetwork including resistors 78 and 80 to the noninverting input ofamplifier 32.

Thus the voltage, and hence the power, applied to those elements of theenabled character matrix selected by the character generator 24 iscontrolled in accordance with the temperature of the center element E ofthe matrix immediately preceding the print cycle so that a print pulseof predetermined length will result in a uniform temperature regardlessof the temperature of the matrix before the print cycle. This procedureis repeated for each character matrix immediately preceding the printcycle for the respective character matrix.

Although a preferred embodiment of the invention has been described indetail, it is to be understood that various changes, substitutions, andalterations can be made therein without departing from the spirit andscope of the invention as defined by the appended claims.

What is claimed is:

'1. In a multiple character electronic display device,

the combination of:

a plurality of character matrices, each of said character matriceshaving a plurality of thermally separated elements each including aheating means,

means for sensing the temperature of each of the character matricesindividually, and

means responsive to said sensing means of a character matrix forselectively applying power to the heating means of the elements of thecharacter matrix at a voltage level related to the temperature of saidcharacter matrix.

2. The combination defined in claim 1 wherein:

each character matrix includes at least one thermally separated elementincluding a semiconductor diode junction in heat exchanging relationshipwith the heating means for sensing the temperature of the charactermatrix.

3. The combination defined in claim 2 wherein:

the diode junction is part of a transistor connected to control currentthrough the resistive heating means, and

the temperature of the diode junction is selectively sensed by closingthe circuit in which the diode is located.

4. The combination defined in claim 3 wherein:

the diode is the base-emitter junction of the transistor, and

the resistive heating means is connected in the collector circuit of thetransistor.

5. The combination defined in claim 4 wherein:

the collectors of the transistors of the elements are completed throughthe resistive heating means of each character matrix to a commoncollector voltage supply line,

the bases of the transistors of the elements are common, and

the emitters of the transistors are separately connectable to an emittersupply voltage.

6. The combination defined in claim 1 wherein said power applying meansincludes:

circuit means for storing a voltage proportional to the temperature of acharacter matrix during a sample period, and

circuit means for applying power that is proportional to the storedvoltage to the heating elements of said character matrix during asubsequent period.

7. In a multi-character electronic display device, the

combination of: V

a plurality of character matrices each comprising a plurality ofthermally separated elements each including a heating element,

first switching means for controlling the current through eachindividual heating element,

temperature sensing means for each matrix for producing a signalrepresentative of the temperature of the respective matrix,

voltage supply means connected to each character matrix including meansfor adjusting the supply voltage in response to a signal from atemperature sensing means,

second switching means for selectively enabling each of the charactermatrices for printing by connecting the voltage supply means to therespective character matrices and respective temperature means, and

circuit means for decoding electrical data representative of charactersand producing outputs for operating the switching means of the enabledmatrix in a manner to heat the elements in a geometric patterncorresponding to the character represented by the character data.

8. The combination of claim 7 wherein:

at least one of the first switching means of each matrix comprises atransistor in heat exchange relationship with the heating element itcontrols, the heating element being connected in the collector circuit,and wherein the temperature of the respective matrix is sensed bypassing a current through the base-emitter junction of the transistorand taking the offset voltage as a measure of the temperature of thematrix.

9. The combination of claim 8 wherein:

the second switching means are in the emitter circuits of thetransistors used as temperature sensors.

10. In an electronic display, the combination of:

a plurality of matrices each comprised of a like number of semiconductorelements,

each semiconductor element including a transistor formed in the elementwith a resistance in the collector circuit for heating the element whencurrent is passed through the transistor,

a separate control line common to the bases of the transistors ofcorresponding elements of the matrices,

switch means for selectively opening the emitter circuits of thetransistors of each matrix to permit enabling of only one selectedmatrix at a time,

means for passing a current through one of the control lines and thebase-emitter junction of the corresponding transistor of the enabledmatrix to sense the temperature of the matrix, and

means for applying power that is related in magnitude to the magnitudeof the sensed temperature to the enabled matrix.

11. The combination of claim 10 wherein:

the means for applying power is circuit means for adjusting the voltageapplied across the resistances and corresponding transistors.

12. The combination of claim 11 wherein the means for applying powerincludes:

means for sampling the offset voltage of the baseemitter junction of theenabled transistor during a sample period and storing the voltage duringa succeeding energizing period, and

means for regulating the voltage applied across the resistances andcorresponding transistors in relation to the stored voltage during theenergizing period.

1. In a multiple character electronic display device, the combinationof: a plurality of character matrices, each of said character matriceshaving a plurality of thermally separated elements each including aheating means, means for sensing the temperature of each of thecharacter matrices individually, and means responsive to said sensingmeans of a character matrix for selectively applying power to theheating means of the elements of the character matrix at a voltage levelrelated to the temperature of said character matrix.
 2. The combinationdefined in claim 1 wherein: each character matrix includes at least onethermally separated element including a semiconductor diode junction inheat exchanging relationship with the heating means for sensing thetemperature of the character matrix.
 3. The combination defined in claim2 wherein: the diode junction is part of a transistor connected tocontrol current through the resistive heating means, and the temperatureof the diode junction is selectively sensed by closing the circuit inwhich the diode is located.
 4. The combination defined in claim 3wherein: the diode is the base-emitter junction of the transistor, andthe resistive heating means is connected in the collector circuit of thetransistor.
 5. The combination defined in claim 4 wherein: thecollectors of the transistors of the elements are completed through theresistive heating means of each character matrix to a common collectorvoltage supply line, the bases of the transistors of the elements arecommon, and the emitters of the transistors are separately connectableto an emitter supply voltage.
 6. The combination defined in claim 1wherein said power applying means includes: circuit means for storing avoltage proportional to the temperature of a character matrix during asample period, and circuit means for applying power that is proportionalto the stored voltage to the heating elEments of said character matrixduring a subsequent period.
 7. In a multi-character electronic displaydevice, the combination of: a plurality of character matrices eachcomprising a plurality of thermally separated elements each including aheating element, first switching means for controlling the currentthrough each individual heating element, temperature sensing means foreach matrix for producing a signal representative of the temperature ofthe respective matrix, voltage supply means connected to each charactermatrix including means for adjusting the supply voltage in response to asignal from a temperature sensing means, second switching means forselectively enabling each of the character matrices for printing byconnecting the voltage supply means to the respective character matricesand respective temperature means, and circuit means for decodingelectrical data representative of characters and producing outputs foroperating the switching means of the enabled matrix in a manner to heatthe elements in a geometric pattern corresponding to the characterrepresented by the character data.
 8. The combination of claim 7wherein: at least one of the first switching means of each matrixcomprises a transistor in heat exchange relationship with the heatingelement it controls, the heating element being connected in thecollector circuit, and wherein the temperature of the respective matrixis sensed by passing a current through the base-emitter junction of thetransistor and taking the offset voltage as a measure of the temperatureof the matrix.
 9. The combination of claim 8 wherein: the secondswitching means are in the emitter circuits of the transistors used astemperature sensors.
 10. In an electronic display, the combination of: aplurality of matrices each comprised of a like number of semiconductorelements, each semiconductor element including a transistor formed inthe element with a resistance in the collector circuit for heating theelement when current is passed through the transistor, a separatecontrol line common to the bases of the transistors of correspondingelements of the matrices, switch means for selectively opening theemitter circuits of the transistors of each matrix to permit enabling ofonly one selected matrix at a time, means for passing a current throughone of the control lines and the base-emitter junction of thecorresponding transistor of the enabled matrix to sense the temperatureof the matrix, and means for applying power that is related in magnitudeto the magnitude of the sensed temperature to the enabled matrix. 11.The combination of claim 10 wherein: the means for applying power iscircuit means for adjusting the voltage applied across the resistancesand corresponding transistors.
 12. The combination of claim 11 whereinthe means for applying power includes: means for sampling the offsetvoltage of the base-emitter junction of the enabled transistor during asample period and storing the voltage during a succeeding energizingperiod, and means for regulating the voltage applied across theresistances and corresponding transistors in relation to the storedvoltage during the energizing period.